A.R. Offringa - LOFAR - CHIME

LOFAR
André Offringa – June 15, 2011
The LOFAR-EoR project
and the LOFAR collaboration
(many slides from Ger de Bruyn and Michiel Brentjes)

LOFAR
(and a little bit of RFI)
André Offringa – June 15, 2011
The LOFAR-EoR project
and the LOFAR collaboration
(many slides from Ger de Bruyn and Michiel Brentjes)

The LOFAR telescope
LOFAR's key properties:
● 10-90 and 110-300 MHz
● Beamformed stations from dipoles, tiles
● Wide field of view
● Very flexible observing modes
● Large collecting area
● Core in the Netherlands
● International baselines up to 1000 km.

The low and high band antennas

Beam former

WSRT
RS307
RS503
CS302
RS208
RS106

1 km
18 core stations
(all split-stations
with 2x24 tiles)
+ 4 more stations
in core
(CS011,013,028,03
1)

Superterp core stations

HBA tiles:
2x24
LBA dipoles:
96
Core stations

International
stations
Effelsberg
Tautenburg 96 tiles

Widefield beam for HBA & LBA
~20
deg
digitally formed station beam
~100
deg
Inner/outer 48
dipoles

Stations are calibrated
● Using redundancy :)

LOFAR observing modes

LOFAR's correlator
● Software correlator (Blue Gene, Groningen)
● Very flexible
● Superb frequency resolution: 0.76 kHz
● Default integration time: 1 sec
● Currently 48 MHz bandwidth, upgrade to 96
(192?) MHz.
● Bandwidth and beams interchangeable, e.g.
24 x 2 MHz beams.

LOFAR key science projects
● Epoch of Reionization!
● Pulsars & transients
● Survey
● Magnetism
● Solar (system)
● Ultra high energy physics / CR

The LOFAR – EoR project
● Exposure/field: 100 x 6h
● 112-190 MHz
● Raw resolution: 1s / 0.76 Khz
● After RFI detection, average time and freq
5-10 times.
● Raw data rate: 30 TB/h
● Averaged data rate: 0.2 – 0.4 TB/h

EoR fields

LOFAR imaging results

3C196 (Wucknitz)

Cas A (Yatawatta)

Cas A (Chandra)

LOFAR offline processing
● Standard pipeline:
– Flags (AOFlagger),
– Averages (NDPPP),
– Subtracts bright sources (demixing),
– Calibrates (BBS),
– Images

LOFAR offline processing
● Offline processing on cluster of ~100
nodes
● Sub-bands are concurrently processed.
● Averaging + flagging takes 25%-50% of
observation time.

● Very fast &
accurate
● Rough estimate of
astronomical
data
● SumThreshold
● Iterative (2.5x)
● Density dilation
AOFlagger

LOFAR and RFI
● RFI environment behaves well
● Few percentage of data requires flagging
● High frequency resolution!
● Antennas on the ground

LBA Total power, before flagging

LBA Virgo, total power, after flagging

HBA “RFI survey” observation, percentages

RFI survey: variance of channel difference (“the noise”)

RFI
● Possible 2 nd stage flagger
● Same algorithm, other input

LBA RFI

HBA RFI

4 observations combined

● LOFAR works
Summary
● RFI environment is benign